Changing in Colors

Chemistry Level 2

Given 4 substances: $AgNO_3$ , $H_3PO_4$ , $NaOH$ and $HCl$ . Mix $AgNO_3$ and $H_3PO_4$ , then add $NaOH$ and finally $HCl$ . In the chemical reaction, we can see the precipitate is yellow after adding $NaOH$ , then white after adding $HCl$ . Put the white precipitate under the sun for a while. We can see it becoming black. What is the chemical formula of black substance?

NaCl

All the other answers are incorrect. It can be any substances because it's burned under the heat of the sun.

Ag_2O

Cl_2

Na_3PO_4

AgCl

Ag

1 solution

Callie Ferguson
Sep 16, 2019

Step 1 : Mix $\ce{AgNO3}$ with $\ce{H3PO4}$

$\ce{AgNO3 + H3PO4 -> 3HNO3 + Ag3PO4}$

Step 2 : Add $\ce{NaOH}$ to the product of the above reaction.

The chemical equation for this reactant would be:

$\ce{3HNO3 + Ag3PO4 + NaOH -> \text{ product}}$ , but $\ce{NaOH}$ will only react with $\ce{Ag3PO4}$ .

Therefore, the resulting (balanced) equation would be:

$\ce{Ag3PO4 + 3NaOH -> Na3PO4 + 3AgOH}$

Step 3 : Add $\ce{HCl}$ to the product of the above reaction.

The chemical equation for this reactant would be:

$\ce{Na3PO4 + 3AgOH + HCl -> \text{ product}}$ , but $\ce{HCl}$ will only react with $\ce{AgOH}$ .

Therefore, the resulting (balanced) equation would be:

$\ce{AgOH + HCl -> AgCl + H2O}$

Step 4: Add sunlight to the product of the above reaction that is white.

Since the only two options are $\ce{AgCl}$ and $\ce{H2O}$ , and we know that $\ce{H2O}$ is clear, we can deduce that the white substance must be the $\ce{AgCl}$ .

Adding sunlight to $\ce{AgCl}$ will cause decomposition, meaning Ag and Cl will separate.

The precipitate of Cl is yellow-green, and the precipitate of Ag is black. Therefore, the chemical formula of the black substance must be $\ce{Ag}$ .

Changing in Colors

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